We have investigated the motion of DNA molecules, driven by electric fields, through nanofabricated systems. By forming systems with dimensions comparable to the size of a DNA molecule, the mobility of the molecules as a function of size can be controlled by the structure. We observed and characterized the trapping and escape of individual molecules in a series of 90 nm-wide constrictions in a nanofabricated channel. The motion of the molecules through the entropic traps exhibited some initially surprising results in the mobility vs. size. These systems can perform the function of DNA separation normally done in organic electrophoretic gels.